Stray current in the water

[JBP-PA]

A friend of mine has found stray current coming from their boat into the water at the starboard engine. This is Lake Erie, so fresh water.
When shore power is not connected, there is no stray current.
When shore power IS connected but their main double breaker is off, there is still the stray current.
When the ground is disconnected from the starboard engine, the stray current goes away.
This suggests to me there is some leakage from hot or neutral to ground and there is a bad connection in the ground circuit somewhere on their boat or possibly in the shore power.

They asked me to help them find it because I am a gearhead. My plan is to take a good volt meter and test the voltage between various ground connections and a "real" ground, perhaps a lead in the water near the problem? Hopefully I'll find zero voltage until I pass the bad spot when I'll see microvoltage. Does this seem like a reasonable plan? Any other tips on how to nail down their problem?

 

[dlochner]

Do they have galvanic isolator installed? If they don't sounds like they need one.

Not connected to shore power and no stray current suggests the shore power ground is the culprit. How much voltage are they measuring?

I think the AC ground and DC- negative should only be connected at one place which could explain why there is no voltage leak from the port engine.

The problem may not be within their boat, except perhaps for the absence of a GI. The classic path for galvanic corrosion is 2 boats docked in the same marina and on shore power. A circuit is formed between 2 dissimilar metals through the water into the AC ground and on to the next boat with an AC ground connection. Interrupting the circuit with a Galvanic Isolator or an isolation transformer takes the boat out of the circuit.

Testing A Galvanic Isolator - Marine How To

A 30A Galvanic Isolator WARNING: The galvanic isolator used in this article does not meet the current safety standards. To meet the current safety standards you would need a "Fail Safe" unit that has been tested to the ABYC Fail

marinehowto.com

 

[jssailem]

If you want to test for stray current in the water, you should use a Corrosion Reference Electrode

Corrosion Reference Electrode with User's Guide

Corrosion Reference Electrode with User's Guide

boatzincs.com

For boat circuits a good digital volt meter will help provide the info regarding connections. Note it may be an AC source not just a DC source.

 

[JBP-PA]

Thanks, I'll ask those questions. BTW, the stray current was identified by the clubs groundskeeper who periodically checks around our boats for stray currents with some device.

 

[jssailem]

I bought one of the reference electrodes. Helped to check my exposed metal thruhulls and shaft.

Additionally, when my diver slips beneath the surface, he’ll share it there is a tingle in the water.

 

[jviss]

That's very scary.